In photolithography, masks are used to expose a pattern upon a semiconductor or wafer for the formation of integrated circuits. One method of forming integrated circuits is by projecting or transmitting light through a mask pattern made of optically opaque or semi-opaque areas and optically clear areas. The optically opaque and semi-opaque areas of the pattern block or partially block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass thereby creating light areas. Radiation is then projected through the mask pattern onto a substrate. Material photoresist of the integrated circuit changes state when exposed to light, forming the integrated circuit.
In lieu of using opaque or semi-opaque areas to form the mask pattern, phase shifting photolithography masks can be used. Phase-shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, or both, thereby changing the phase or the periodic pattern of the light wave. Phase shift masks reduce diffraction effects by combining both diffracted light and phase shifted diffracted light so that constructive and destructive interference takes place favorably.
A third method of forming integrated circuits is by combining the two photolithography mask methods discussed above. A mask pattern therefore would consist of phase shifting techniques and opaque or semi-opaque areas. Regardless of the patterning method used on a mask, photolithography utilizes a beam of light, such as ultraviolet (UV) radiation, through an imaging lens to transfer the pattern from the mask onto a photoresist coating layered upon the semiconductor wafer.
Each of the above described methods rely upon the physical properties associated with the materials used in forming the masks. Once a mask is formed, it is a permanent structure that can not be easily changed. Three masks using the above discussed techniques are illustrated in FIGS. 1A-C. All three masks use a quartz 21 structure upon which a pattern is formed. The masks comprise: 1) material that block or partially block the light, or 2) a notch in the quartz substrate to change a phase of the light, or 3) a combination of the notch and the material.
Referring to FIG. 1A, a first prior art mask 20 uses an etched layer 22 of quartz 21 with opaque materials 24 deposited on the underside. As light passes through the quartz 21 section represented by arrow 30, a 100% transmission of light passes through without a shift in phase. A 100% transmission of light through a quartz/air or an air/quartz interface assumes that no light is lost or blocked. In actuality, some light is blocked by the transition between these interfaces. Typically, the amount of light that passes through a quartz/air interface or an air/quartz interface is approximately 92% with light having a wavelength of 248 nanometers. For purposes of discussion herein, 100% is used in lieu of actual percentages that may vary depending on the wavelength of light.
Similarly, as light passes through the notched 22 section represented by arrow 32, a 100% transmission of light passes through, except with a 180 degrees shift in phase. For purposes of discussion herein, 180 degrees is used in lieu of actual degrees that may vary depending on the characteristics of regions causing the phase shift. The notch 22 creates the phase shift. When light hits opaque material 24, as represented by arrow 31, the light is completely blocked.
A second prior art mask is illustrated in FIG. 1B, where semi-opaque material 40 is deposited on the underside of the mask 26. When light passes through the semi-opaque material, as represented by arrow 42, the semi-opaque material 40 blocks some of the light, e.g., 10%, and shifts the phase of the light that does pass through. The shift in phase is 180 degrees from the light that passes through the quartz section 21, as represented by arrow 44.
Finally, the third type of prior art mask is illustrated in FIG. 1C, where the quartz 21 material is notched 54 to change the phase of the light passing through. Both sides transmit 100% of the light. However, the notched side represented by arrow 50 produces a 180 degree phase in light as compared to the light transmitted through the un-notched side represented by arrow 52.
A common feature of these three masks is that they are all designed to mechanical specifications to produce the desired patterning effect. A common disadvantage is that once a mask is produced, it cannot be easily changed. To make a change in the pattern often requires a new mask to be manufactured. Changing a design is both costly and time consuming because of the lengthy steps required to reconstruct a mask. Accordingly, this disadvantage discourages experimentation. Another disadvantage of the masks illustrated in FIGS. 1A-C is that the physical characteristics degrade over time, thus effecting optical performance.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to perform photolithography with a masking plate that is easy to change and its performance does not degrade over time.